Color electrophotographic printing is becoming popular for in-house publishing and other applications. To achieve a high quality image, printing media selected for use can play an important role.
Paper blister is common problem that can occur with many coated papers used for electrophotographic printing, typically due to fusing processes required in such printing procedures. Coated papers typically include a hydrophilic paper base that is coated with pigment particulates. When toner images are transferred onto the paper, and the paper/toner combination undergoes fusing, the water absorbed inside the media is typically heated causing vaporization, thereby generating strong vapor pressure beneath the coating. If the vapor pressure cannot dissipate relatively instantly and homogenously along the paper surface, a “popcorn” effect can occur that causes blistering. This can be further exacerbated if the media was prepared or printing occurs under higher humidity conditions. Further, if multiple heated fuser rollers are used, or higher fusing temperatures are present to achieve high toner gloss, blistering can be even more pronounced. Additionally, when duplex printing using media that is coated on both sides, it is even more difficult for water vapor to escape from the media.
Another problem associated with electrophotographic media is related to image defect caused by electrostatic discharge, especially when printing is carried out at a low temperature and/or a low relative humidity condition. This defect can produce uneven color density, particularly in lower toner density areas. The mechanism of this defect caused by an undesirable electrostatic discharge is not fully understood. It can be related to surface smoothness and thickness of the media, where rough surfaces and thicker media usually exhibit more significant defects than smoother and/or thinner media, but it is also related to the resistivity of the media. The electrical resistivity of the media, with an optimum value of 1×109 to 1×1011 ohm/square, plays a dominant role in controlling image quality. When the surface resistivity is below 1×108 ohm/square, an insufficient amount of toner can be transferred to the receiving layer, leading to low optical density print images. When the surface resistivity exceeds 1×1013 ohm/square, an excessive amount of electrostatic charge can be generated during toner transfer and paper transport, which can lead to uneven toner density caused by electrical discharge. Additionally, the speed of charge dissipation on the media surface, known as electrical relaxation, is also related to image defect caused by electrostatic charge. Thus, an appropriate relaxation time can be desired, keeping in mind that too long of a dissipation time can result excessive electrostatic building-up.